This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. The goal of this study, as a component of the NIH-sponsored Rare Diseasese Clinical Research Network (RDCRN), is to obtain detailed longitudinal natural history data on a large population of individuals affected by Prader-Willi syndrome (PWS) and, to a lesser extent, individuals who have Early Morbid Obesity (EMO) not due to PWS. The ultimate hope is that this well-studied population will provide data to be used in future for diagnostics, therapies and genotype-phenotype correlations for these two diseases. Prader-Willi syndrome (PWS) is a complex neurobehavioral syndrome whose main clinical features include obesity, hyperphagia, hypotonia, cognitive impairment, a distinct behavioral phenotype, hypogonadism, and neonatal failure-to-thrive. The genetic defect has been localized to a 2 megabase region of chromosome 15q11-q13. It is a contiguous gene syndrome with the loss of expression of several genes resulting in the complete phenotype. As PWS is the most common recognized genetic cause of morbid obesity, a growing number of morbidly obese children are being referred to pediatric genetics and pediatric endocrinology for evaluation of PWS. Many of these children clinically and molecularly do not have PWS. We refer to these individuals as the Early Morbid Obesity (EMO) population.